Research on the utility of repellents in reducing the impact of spotted-wing drosophila (Drosophila suzukii) on fruit crops found that one compound, octenol, shows potential. Another, geosmin, fared well in lab tests but had little efficacy in field experiments. (Photo credit: Hannah Burrack, North Carolina State University, Bugwood.org)

By Edward Ricciuti

Cornell University scientists are tuning up the entomological version of psych war tactics that, instead of killing insect pests outright, manipulate their behavior so they avoid crops they might otherwise damage. Repellents that modify the behavior of insects might not stand alone as deterrents, but they could be deployed more extensively in integrated pest management (IPM) than presently, according to Anna Wallingford, Ph.D., of Cornell’s Department of Entomology.

Ed Ricciuti

Wallingford and colleagues have conducted research to pinpoint hallmarks of repellent stimulants practical enough for use on crops and to increase their role in IPM. Their perspectives were published in July the journal Environmental Entomology.

With fellow researchers at Cornell and the U.S. Department of Agriculture, Wallingford tested the impact of two chemical repellents on egg laying in fruit by Drosophila suzukii, also known as spotted-wing drosophila (SWD),an invasive Asian relative of the well-known fruit fly. SWD does double damage to fruit, penetrating the skin with its ovipositor, edged like a serrated knife, and again when larvae feed. A single female may hit the same fruit several times, depositing two or three eggs each time. Making matters worse, SWD lays eggs just before berry and fruit crops ripen. Since crop maturity times differ, SWD may damage several crops in the course of a season, particularly before autumn harvest, when the fly thrives in cool temperatures.

The chemicals tested, both odoriferous alcohols, were octenol—found in mushrooms, some aromatic cheeses, raspberries, and several other foods, as well as some animals—and geosmin, made by certain bacteria and algae and responsible for the earthy taste of beets and smell of newly plowed soil. (In fact, the word “geosmin” derives from Greek for “earth smell.”) Both somewhat repel SWD. The manner in which octenol does so is uncertain.  Geosmin triggers an automatic avoidance response by SWD, in which sensory nerves directly signal motor nerves to put the fly in reverse.

Wallingford and her colleagues have been looking for repellents that can act on different stages of behavior. They filmed the entire behavioral repertoire leading to egg laying by SWD flies caged and in the field, exploring various ways in which they responded to the odor of dispensed repellents. In effect, they took the position that it is not enough to know a repellent works; instead, they sought to know how it works at all stages of behavior, both before and after contact with the fruit and in both laboratory and field. Stopping a pest before it makes contact with a fruit, for example, prevents it from spreading disease as well as laying eggs. The most promising repellents, the researchers conclude, are those that act on as many of an insect’s sensory systems as possible.

The research is part of an ongoing effort at Cornell to identify repellents that do not have to be applied directly to fruit itself and can be used in conjunction with other controls. If applied to the surface of fruit, aromatic repellents could impact taste, especially if right before harvest, which would be necessary in the case of SWD.

“I don’t think anyone would like a raspberry that tasted like mushrooms or soil,” says Wallingford.

Octenol and geosmin were tested alone and combined. The scientists found that, in the laboratory, each repellent curbs egg laying but in different ways. Compared to control fruits, those treated with geosmin treatment decreased the number of eggs laid by females but not their visits to fruits. The reverse was true for octenol.

Geosmin seemed to make no difference in the number of females that stuck around on fruits after landing, but they switched between treated fruits more frequently. On the other hand, few females remained when octenol was present, and those that stayed switched less frequently. In the field test, researchers found fewer eggs in fruits protected by octenol, both alone and when combined with geosmin, but the number was the same in fruits treated only with geosmin as in controls.

The scientists suspect that octenol interferes with the ability of SWD to locate and land on fruits, perhaps by the fly’s response to attractive stimuli. Geosmin, on the other hand, seems in the laboratory to reduce egg laying itself after the fly lands, although how it does so is unclear. In the field, however, geosmin loses its power to do so and, say the scientists, is not a practical option for IPM.

“One important take-home message from our experimental results with geosmin is to avoid the assumption that stimuli inducing the most conspicuous effects in the laboratory will be the most useful tools when scaled up for use in the field,” say the researchers. Octenol looks viable for field use, and not just for SWD. “Most insects detect octenol, so there is potential for confusing the searching behavior of other insect species,” says Wallingford.

Octenol can be a two-edged sword, however. Found in human breath and sweat, it attracts arthropods that feed on mammals, including mosquitoes and ticks.

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.